Many utility companies have introduced programs to encourage energy conservation among their customers. An electric company considers placing electronic meters in households to show what the cost would be if the electricity use at that moment continued for a month. It gives these meters to 100 of its customers for a year and then compares the average electricity use in these customers' homes this year to the previous year. Result: These customers' average electricity use decreased by 10 % . Explain why this is not strong evidence that the use of the electronic meters caused customers to decrease their electricity use.